The prevention of Si pitting in hydrofluoric acid cleaning by additions of hydrochloric acid

Bare Si surfaces, when immersed in hydrofluoric acid (HF), are susceptible to oxidizing metallic impurities such as Cu+2 and undergo a galvanic reaction. The Si is oxidized and dissolves in the HF solution, while the Cu+2, for example, is reduced to metallic Cu. The manifestation of the Si oxidation is the generation of pits or roughened surfaces, depending on the level of metallic contaminants present in the HF. We investigated the pitting effects of several metallic contaminants (Cu+2, Hg+2, Ag+, Fe+3, Ti+4, V+5, and Cr+6) intentionally added to a concentrated HF (1:1 HF/H2O) mixture. All created pits on bare Si. However, adding hydrochloric acid (HC1) to the HF suppressed the pitting or surface roughening phenomena of Cu+2, Hg+2, and Ag+ contaminants, making the process immune to metallic impurities in the HF. Gate oxide test structure results indicate no adverse effect of adding HCl to HF.